What people don't think about incest is how serious it is for future offspring. Many leave with health problems, malformations and mental retardation. A very common and documented case was a royal dynasty of Spain, the Austrians.
Those cases are close relative incest repeated in many generations.
Genetic diseases are mostly based on recessive allele. As allele are inherited, if you have one, there is an increase risk that a member of your family have it too (this increasing risk decrease rapidly with kin level, that's why if you commit incest every few generation with strangers in between it's not so bad). Even if you and your partner have one, the risk to have a child with the disease (who have two bad allele) is one on four. To say "incest automatically mean genetic disease " is far from reality. There is statistically a bit more risk than when you fuck a stranger, but not that much, as those allele are rare anyway. And it also increase by same amount the chance to have a good recessive character. Plus, today, we know how to detect such allele with genetic screening and can suggest to people who have some bad allele and are at risk to use contraception and choose alternative methods of procreation.
In Island, where everyone is more or less relative (small population, almost no immigration), they do a large study about it, using their census archives. It shows that if we consider number of children and life expectancy, people winning darwinian competition were those who marry their second cousin, so a bit relative. People who repeat close incest on many generations sure were the looser, but not as much as previously believed.
Also today sex is largely disconnected from procreation. We do it for pleasure. So we can do incest this way too.
I was under the impression that Rhode Island was actually more "friendly" in the respect that everything, including parental incest, is legal there as long as all parties are of age.
Rhodes Island and New Jersey are the two american states where consensual adult incest is legal to my knowledge. It's also the case in many western european countries. Unfortunately, legal doesn't mean socially accepted.
Historically, there were countries like ancient Egypt or middle age Persia were incest was not only legal but considered sacred and encouraged. In late ancient Egypt, a census registry that archeologists found in norther Egypt show 1/4 people were incest married, ie almost everyone with a relative of almost same age choose to do it. Those two countries are also some of the most brilliant of their period.
There is plenty of misconception of this subject, that came from ignorance and thousands years of biased theories invented by some anthropologists blinded by ethnocentrism and anti incest religious propaganda.
Here if you want, a text i write on the subject, gathering some knowledge i found when studying it:
